Extract for-of iterator handling to a helper (#11262)

* Extract for-of iterator handling to a helper

Dis greatly recudes the code size when for-of is used multiple times across the
codebase. Also, makes it easier to read what's happening in a compiled loop.

* Unify spec and loose code

* Legacy implementation fallback

* Update tmp var name

* Updates from review and after rebase

packages/babel-plugin-transform-block-scoping/test/fixtures/general/hoisting/options.json

@@ -4,6 +4,7 @@
     "syntax-jsx",
     "transform-react-jsx",
     "transform-block-scoped-functions",
-    "transform-for-of"
+    "transform-for-of",
+     ["external-helpers", { "helperVersion": "7.100.0" }]
   ]
 }

packages/babel-plugin-transform-block-scoping/test/fixtures/general/hoisting/output.js

@@ -10,12 +10,11 @@ var _loop = function (i) {
   });
 };
 
-var _iteratorNormalCompletion = true;
-var _didIteratorError = false;
-var _iteratorError = undefined;
+var _iterator = babelHelpers.createForOfIteratorHelper(nums),
+    _step;
 
 try {
-  for (var _iterator = nums[Symbol.iterator](), _step; !(_iteratorNormalCompletion = (_step = _iterator.next()).done); _iteratorNormalCompletion = true) {
+  for (_iterator.s(); !(_step = _iterator.n()).done;) {
     var i = _step.value;
     var x;
     var f;
@@ -23,16 +22,7 @@ try {
     _loop(i);
   }
 } catch (err) {
-  _didIteratorError = true;
-  _iteratorError = err;
+  _iterator.e(err);
 } finally {
-  try {
-    if (!_iteratorNormalCompletion && _iterator.return != null) {
-      _iterator.return();
-    }
-  } finally {
-    if (_didIteratorError) {
-      throw _iteratorError;
-    }
-  }
+  _iterator.f();
 }